After searching the web (the usual suspects http://stackoverflow.com/ and Google) and trying a lot of the given hints, I finally tried to checkout the trunk of the repository with subversion directly and did not succeed as well.
Now I firstly was even thinking our subversion repository might be corrupt until a sudden insight grew in me: while I really like the Macbook Pro given to me by the company, the Avira virus scanner was detecting the EICAR test virus in the test suite, this showed up in the quarantine logs!
Now the rest was a piece of cake: by applying the hints on the Avira KB for excluding specific paths I finally succeeded by excluding only this specific directory where the conversion took place.